1. Field of the Invention
The present invention relates to a low-resistance MOS field effect transistor (hereafter referred to as MOSFET) used for power circuits and the like based on the synchronous rectification.
2. Description of the Related Art
In recent years, power circuits based on the synchronous rectification are extensively used according as a low-voltage power is used for CPUs in computers and the like. This power circuit conventionally uses a trench MOSFET having the trench gate structure.
A conventional low-resistance MOSFET is described with reference to FIGS. 1A and 1B.
FIG. 1A is a sectional view showing a configuration of a conventional trench MOSFET. The trench MOSFET comprises a gate electrode 201, a source electrode 202, and a drain electrode 203. To achieve low on-resistance, this trench MOSFET employs a trench gate which uses as a channel the side wall of a trench buried with the gate electrode 201.
However, the trench MOSFET in FIG. 1A allows the gate electrode 201 to directly contact with a drain layer 205 through the intermediation of a thin oxide film 204, causing a large parasitic capacitance between the gate electrode 201 and the drain layer 205. Accordingly, that trench MOSFET is inappropriate for high-frequency switching.
A planar MOSFET as shown in FIG. 1B is used as a high-speed switching element suitable for the high-frequency switching. This planar MOSFET has a gate electrode 211, a source electrode 212, and a drain electrode 213. However, the planar MOSFET offers a problem of large on-resistance.
When the MOSFET is used under an inductive load, there is a disadvantageous effect that applying a voltage exceeding the element's withstand voltage causes avalanche breakdown and destroys the element.